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Protesters in several U.S. cities blocked highways and swarmed police precincts, leading to at least two dozen arrests in demonstrations touched off by fresh cases of police violence against unarmed black men.

Marching across New York's Brooklyn Bridge, some 250 placard-bearing activists organized by the Stop Mass Incarceration Network protested the latest incidents of violent police tactics used against minorities.

Hoisting signs reading "Stop murder by police" and "Stop killer cops," they said they hoped to spur national discussion on the issue. At least 12 people, some of whom appeared to be school-aged, were arrested following a brief scuffle with police after they crossed the bridge.

Police in Los Angeles said they arrested 15 protesters in a group of nearly 100 after they stopped on Metro Rail tracks and ignored orders to disperse.

Elsewhere on the West Coast, more than 100 protesters in San Francisco surrounded a police station and disrupted a meeting at City Hall of the Board of Supervisors. In nearby Oakland, demonstrators massed outside the Oakland Police Department and swarmed onto Interstate 880, according to local television images.

Sign-waving protesters marched through downtown Seattle, briefly blocking commuter traffic at various points, though the demonstration was peaceful and there were no arrests, police and transit officials said.

And in Wisconsin, about 100 protesters, mostly high school students, blocked a major roadway in Madison, where last month's fatal shooting of unarmed black teen Tony Robinson Jr. by a white police office has triggered a series of demonstrations.

Galvanizing their cause was the April 4 fatal shooting of Walter Scott, an unarmed black man shot in the back by a white police officer in North Charleston, South Carolina.

The shooting was captured on video, and the officer has been charged with murder.

Family members of several unarmed black or Hispanic men or boys who died in encounters with police called for more oversight.

"What this protest right here is about is that too many are being murdered," said Nicholas Heyward Sr., whose son Nicholas Heyward Jr. was shot dead at age 13 in public housing by a police officer 20 years ago while playing cops and robbers with a toy gun.

"I have been fighting for the last 20 years to get that case reopened," he said.

"It's painful because not only do I have to wait but while I'm waiting, I am constantly seeing innocent victims gunned down on the street for no reason at all."

Last year, a tide of protests was inspired by a string of high-profile cases of black men losing their lives at the hands of white police officers.

But the outbursts of anger following the deaths of Michael Brown in Ferguson, Missouri, and Eric Garner in New York slowed to a standstill over the winter.

Another group of protesters, led by Justice League NYC, has embarked on a 250-mile trek to Washington from New York City to demonstrate against police-related deaths.